Generically, focusing rail. Velbon's isn't the only one. For example, Novoflex offered them decades before Velbon started in business.

In my experience, a focusing rail isn't necessary when shooting an SLR with flash at magnifications not much higher than 1:1. At higher magnifications or using available darkness for illumination, very very useful. I use a 2-axis Panagor -- looks much like the 2-axis one that Adorama sells but is better made and has more locks -- under my Graphics when shooting one of them very close.